More precisely, the present invention relates most particularly, but not exclusively, to measurement systems of the type shown in FIG. 1 comprising both a resistive sensor Ca whose output resistance is a function of a detected physical magnitude, and an associated gauge Log, e.g. of the ratio meter type, connected to the output of the sensor Ca.
The resistive type sensor Ca, e.g. of the rheostat type, may have an output resistance that depends, for example, on the level of a liquid, such as the level of a fuel.
In known manner, ratio type gauges Log generally comprise at least two geometrically crossed coils B1 and B2. The coils B1 and B2 are connected electrically in series, generally between a ground terminal and a positive power supply terminal +Bat. The common point between the two coils B1 and B2 is connected to the output of the sensor Ca.
A bipolar permanent magnet secured to a pointer is mounted to rotate in the field of the two coils B1 and B2. Thus, the magnet and the pointer of the meter move as a function of the ratio between the currents flowing in the two coils B1 and B2, tending to come into alignment with the direction of the magnetic field that results from combining the fields in the two coils B1 and B2.
Numerous gauges have already been made that satisfy the above definition.
In particular, numerous gauges for the level of a liquid, in particular a fuel, have been made that satisfy the above definition. In the context of this particular application to measuring a liquid level, the sensor Ca is generally constituted by a resistive track over which there moves a wiper that is actuated by a float which tracks variations in the level of the liquid.
The person skilled in the art knows that the level of fuel in a motor vehicle tank is highly sensitive to accelerations and decelerations of the vehicle.
Consequently, various means have already been devised for avoiding that such untimely fluctuations in the measured liquid level give rise to oscillations in the reading given by the gauge.
As described in document FR-A-2 561 379, proposals have been made, for example, for devices that apply mechanical damping to the float sensor Ca. More precisely, according to that document, proposals have been made to mount the float that controls the sensor Ca at a first end of a lever and to constrain the second end of the lever to rotate with an inlet gear that drives a multiplicative gear train constituting the shock absorber device.
As described in document FR-A-2 592 479, for example, proposals have also been made to apply electrical damping to the signal output by the sensor Ca. As shown diagrammatically in FIG. 2, devices applying the teaching of that document include, inter alia, an analog lowpass filter PB interposed between the output from the sensor Ca and the input to the ratio meter Log, i.e. the common point between the two coils B1 and B2. More precisely, such a lowpass filter PB comprises a series resistor R1 and a parallel capacitor C1 connected to ground.
Still more precisely, as shown diagrammatically in FIG. 2, the electrical damping circuits Cae interposed between a sensor Ca and a ratio meter Log generally include a driver circuit referenced "PWM" (for Pulse Width Modulator) in FIG. 2 which applies a periodic squarewave electrical signal to the input of the ratio meter Log, with the duty ratio thereof depending on the amplitude of the signal delivered by the lowpass filter PB.
A very large number of gauges applying the teaching of said document FR-A 2 592 479 and shown diagrammatically in accompanying FIG. 2 have already been sold.
However, those devices still do not give complete satisfaction.
In particular, the Applicant has observed that the capacitors C1 of the analog lowpass filters PB give rise to non-negligible leakage that have the effect in practice of reducing the value and the stability of the time constant that can be achieved.
Furthermore, placing such a damping filter Cae, PB between the sensor Ca and the gauge Log has required gauges Log to be implemented having special characteristics such that said gauges are no longer suitable for use directly with a conventional sensor Ca, i.e. without a filter Cae, PB. It will be understood that the use of such special means gives rise to inevitable problems of supply and part number identification.